The gist of the argument is that Piers Morgan, like Trump and the rest of the right wing press, thinks that protestors are being hysterical. Everywhere you look at the moment the word is being evoked, 'hysterical protestors', 'hysterical reaction to the Trump administration', 'hysterical rhetoric' etc. etc. etc.
Today, branding a person or a movement as hysterical is a way to pathologize emotion. What's more, by applying ‘hysteria’ to Owen Jones and other male protestors, Piers is attempting an attack on their masculinity. He’s the bully in the playground who’s just called the other guy a pussy.
Of course, feminising someone or something you don’t like as an insult is not exactly new, but what’s interesting about this particular example is the insidiousness of it. My old French teacher once joked that when it came to gendering nouns “the bad things are usually feminine” and like gendered nouns, the sexism at play here is of the unembodied, 'read between the lines' variety. Accusing protestors of hysteria is a way of devaluing righteous anger along gendered lines.